Analog-to-digital converters (ADC) have been implemented using a voltage controlled oscillator (VCO) wherein an input voltage is converted to an output frequency. The output frequency may then be converted to a digital value by driving a counter over a sampling interval. The counter output may then represent an average of the input voltage over the sampling interval. The accuracy of this type of ADC is affected by the center frequency (gain) of the VCO which may drift over changing environmental conditions (e.g., temperature), non-linearity of the VCO transfer function, and quantization error of the digital counter.